Gaseous alternative fuels, such as hydrogen and natural gas, are valued for their clean burning characteristics in motor vehicle engines. Various processes have been developed for producing hydrogen. These processes include electrolysis, exotic water splitting, and separation from industrial waste streams.
Hydrogen can also be produced by reforming natural gas. Typically, a multi-step process is used to convert a hydrocarbon fuel, such as methane, propane or natural gas, into a high purity hydrogen gas stream. The steps of the process typically include (1) synthesis gas generation, (2) water-gas shift reaction, and (3) gas purification (e.g., CO and CO2 removal). The hydrogen gas stream can then be used for a variety of purposes including mixture with other gases to produce an alternative fuel.
For example, a particularly clean burning gaseous alternative fuel known as HYTHANE comprises a mixture of hydrogen and natural gas. The prefix “Hy” in HYTHANE is taken from hydrogen. The suffix “thane” in HYTHANE is taken from methane, which is the primary constituent of natural gas. HYTHANE is a registered trademark of Brehon Energy PLC. HYTHANE typically contains about 5% to 7% hydrogen by energy, which corresponds to 15% to 20% hydrogen by volume.
For producing hydrogen, one type of reformer called a “steam reformer” uses a hydrocarbon fuel and steam (H2O). In the steam reformer, the hydrocarbon fuel is reacted in a heated reaction tube containing steam (H2O) and one or more catalysts. In general, the production of a high purity hydrogen gas by reforming requires high temperatures (800-900° C.). Steam reforming also produces impurities, particularly CO and CO2, which if not removed, are ultimately released to the atmosphere.
The production of a high purity hydrogen gas by reforming also requires large capital costs for the equipment, and large operating costs, particularly for power. In addition to these shortcomings, it is difficult to manufacture a compact embodiment of a steam reformer. It would be advantageous for a hydrogen production system to have a relatively compact size, such that alternative fuels could be produced at a facility the size of a gas station, rather than at a facility the size of a refinery.
Another process for producing hydrogen from natural gas involves the thermal decomposition of methane. For example, methane decomposes into hydrogen by the reaction:CH4═C+2H2 For example, the thermal decomposition of natural gas has been used in the “Thermal Black Process” for producing carbon black and hydrogen. Using thermal decomposition, the energy requirements per mole of hydrogen produced (37.8 kJ/mol H2) is considerably less than the energy requirements of the steam reforming process (63.3 kJ/mol H2). However, the process still requires high temperatures (e.g., 1400° C.), high equipment costs, and high energy expenditures.
Recently, thermal decomposition of natural gas has been investigated in combination with various catalysts, which allow the reaction to proceed at lower temperatures. For example, U.S. Pat. No. 7,001,586 B2, to Wang et al. discloses a thermal decomposition process in which two catalysts having the formula NixMgyO and NixMgyCuzO, respectively, are used to decompose methane to carbon and hydrogen. The former needs a lower temperature from about 425° C. to 625° C., but the lifetime is shorter and the activity is lower. The latter's lifetime is longer and the activity is higher, but the required reaction temperature is much higher, from about 600° C. to 775° C. More importantly, however, these processes require high energy expenditures to heat the wall of the reactor, the gas and the catalysts.
Methane plasma has been used to convert methane into C2 (such as C2H2, C2H4, C2H6) and hydrogen. When microwave plasma is combined with a metal catalyst, the metal catalyst is heated by microwave energy. The combination of methane microwave plasma with the metal catalyst as reported in the literature so far, can efficiently convert methane to C2H2, C2H4 and C2H6 and H2 is a by product. But the product gas comprises C2 and H2 with a stoichiometric relation, and can not be directly used as HYTHANE due to the high concentration of C2. In addition, the prior art catalysts are sensitive to carbon deposition, which deactivates the catalyst and decreases the production of both C2 and H2.
It would be advantageous for a hydrogen production system to be capable of performance at lower temperatures and lower energy expenditures, with a variety of catalysts active for long periods, and with minimal carbon emissions (e.g., CO, CO2) and negligible higher order hydrocarbons. In addition, it would be advantageous for a hydrogen production system to have a size and configuration adaptable to the production of alternative fuels containing hydrogen. The present disclosure is directed to a method and a system for producing a hydrogen enriched fuel that overcomes many of the shortcomings of prior art hydrogen production systems.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. Similarly, the following embodiments and aspects thereof are described and illustrated in conjunction with a system and method, which are meant to be exemplary and illustrative, not limiting in scope.